Conventional loudspeakers use a voice coil connected to a sound radiating cone or dome, with the voice coil being immersed in the field of a permanent magnet. An electrical audio signal is passed through the coil, causing linear motion of the coil and vibrations in the radiator.
The efficiency of a conventional linear drive loudspeaker is related primarily and linearly to the mass of the moving components, the radiating area, and the efficiency of the solenoid motor. However, even with the use of light weight materials in the cones and coil, and the use of strong magnets, the potential efficiency of conventional speakers is very low and is usually less than a few percent. Moving mass constraints impose a limit on the amount of wire which may be used in the voice coil.
Departures from conventional technology may be found in U.S. Pat. Nos. 4,564,727 and 4,763,358. In both patents, the shaft of a separate commutated electrical motor is connected to a radiator to generate sound. In U.S. Pat. No. 4,564,727, the rotary motion of the motor is converted to linear motion to drive a radiator. In U.S. Pat. No. 4,763,358, the shaft of the motor is connected to the shaft of a rotary vane disposed in tube having a baffle. The tube has a port on one side of a baffle through which sound is emitted. These devices are most suitable for a production of low frequency sound at high intensities, since excursions of the motor are unlimited, and the separate motor has a higher efficiency than a voice coil and magnet drive.